1. Field of the Invention
The present invention relates to lithium metal composite oxide particles, a process of producing the lithium metal composite oxide particles, an electrode structure containing the above-mentioned lithium metal composite oxide particles, and a process of producing the above-mentioned electrode structure, as well as to a lithium secondary battery which has the above-mentioned electrode structure. More specifically, the present invention relates to lithium metal composite oxide particles having excellent properties usable as an electrode material in a lithium secondary battery, a process of producing the lithium metal composite oxide particles, an electrode structure containing the above-mentioned lithium metal composite oxide particles, and a process of producing the above-mentioned electrode structure, as well as to a lithium secondary battery which has a positive electrode constituted of an electrode structure composed of the above-mentioned lithium metal composite oxide particles. The lithium secondary battery has a high capacity and excellent high-rate discharge characteristic and charge/discharge cycle characteristic.
2. Related Background Art
The amount of CO2 gas contained in the atmosphere has been recently increasing, and global warming due to the greenhouse effect is worried about. In the meantime, thermal power plants that convert a thermal energy obtained by combustion of fossil fuel into an electric energy discharge a large quantity of CO2 gas. It is becoming difficult to newly build a thermal power plant under these circumstances. Taking such a situation into consideration and for the purpose of coping with a growing demand for electricity, the so-called load leveling approach has been proposed as a method of utilizing electric power effectively, in which approach an electric power is stored during night in secondary batteries installed at individual houses and is used in the daytime for much power consumption thereby equalizing the load. Besides this, motorcars running by fossil fuel and discharging CO2 gas, NOx, SOx and the like are regarded as problematic as another source of air pollutants. From a viewpoint of reducing the sources of air pollutants, electric vehicles that run by powering a motor with electricity stored in a secondary battery and discharge no air pollutants draw attention, and research and development are intensively conducted so as to early put them in practical use. The secondary battery used for such electric vehicles is required to have high energy density, a long life and low cost.
Apart from this, as a secondary battery for use in the power supply of a portable apparatus such as a notebook personal computer, a word processor, a video camera, and a cellular phone, early provision of compact, lightweight and higher-performance secondary batteries are eagerly desired.
As such a compact, lightweight, and high-performance secondary battery, an example of applying a lithium-graphite intercalation compound to a negative electrode of a secondary battery was reported in JOURNAL OF THE ELECTROCHEMICAL SOCIETY 117, 222 (1970) and since then, a rocking chair type secondary battery, the so-called “lithium ion battery” which, for example, uses carbon (including graphite) for a negative electrode active material and lithium cobalt oxide particles for a positive electrode active material and inserts and occludes lithium between carbon layers via a charging reaction have been developed and partly put into practical use.
In the meantime, as a positive electrode active material for the lithium secondary battery including such a “lithium ion battery”, lithium cobalt oxide LiCoO2 has been mainly used. Further, as a positive electrode active material other than the above-mentioned lithium cobalt oxide, use of lithium nickel oxide, lithium manganese oxide, or the like is being studied. Moreover, those compounds having a part of the metallic elements of these oxides substituted with other metallic elements have also been proposed and partly used. When a positive electrode active material is to be selected, selection is performed in consideration of advantages and disadvantages in terms of cost and performance and depending on the purpose. In relation to the potential performance which an positive electrode active material originally has, the constitutional elements and crystal structure of the active material are important but in the lithium secondary battery which uses the so-called composite electrode formed by mixing with a conductive auxiliary material and a binder, the shape of active material particle to be used is also an important factor. For example, the smaller the particle size is, the larger the specific surface area tends to be, and an active material particle with a larger specific surface area has a larger surface area in contact with a nonaqueous electrolytic solution thereby providing an advantage to allow a uniform electrochemical reaction in charging/discharging. However, if the particle size of the active material is too small, unless the amount of a binder used is increased, the binding strength will become weak, whereby the active material will become easy to peel off from the electrode. On the other hand, if the amount of the binder is increased, the active material particles will be excessively covered by the binder thereby reducing the conductivity, so that there arises a necessity to increase the amount of the conductive auxiliary material used. Further, if the amount of the conductive auxiliary material used is thus increased, the binding strength will become weak to effect easy peeling off of the active material, so that the electrode formation will become difficult. On the contrary, if the size of the active material particle is simply too large, the specific surface area thereof will become smaller, and as a result, there will be posed the problem that a nonaqueous electrolytic solution may reach the inside of the active material particle in a prolonged time and consequently uniform electrochemical reaction during charging/discharging may be hindered, whereby the original performance of the active material cannot efficiently be exhibited.
Under these circumstances, a positive electrode active material for use in a lithium secondary battery is desired to have a moderately large particle size suitable for electrode formation as well as a larger specific surface area in order to make advantageous for charging/discharging. Further, there is an eager need for providing a lithium metal oxide particle that has such a shape as to meet such a need and a process of producing the same.
Japanese Patent Application Laid-Open No. 5-290849 discloses a process of producing lithium metal oxide particles having a large average particle size usable for a positive electrode active material in a nonaqueous electrolyte secondary battery, by charging lithium carbonate and cobalt oxide powders into a granulator, adding a binder thereto, granulating the mixture and calcining the obtained granules. Japanese Patent Application Laid-Open No. 10-74516 discloses a process of producing lithium cobalt composite oxide (LiCoO2) particles usable for a positive electrode active material for a lithium secondary battery by dissolving lithium nitrate and cobalt nitrate in a mixed solution of water and alcohol, spraying the resulting mixed solution as a mist from a nozzle into a thermal decomposition furnace adjusted to a predetermined temperature at a predetermined rate, thermally decomposing the mixture to obtain a composite oxide powder and annealing the composite oxide powder at a predetermined temperature. The Laid-Open gazette discloses that the particle of the above-mentioned lithium cobalt composite oxide has a hollow spherical shape formed of secondary particles which are in turn formed of aggregated primary particles and that the secondary particles have an average particle size of 1 to 5 μm and a specific surface area of 2 to 10 m2/g.
However, the active material particles prepared by the above-mentioned processes have the problem that the specific surface area is not sufficiently large, and the contact resistance between the active material particles is large and consequently the capacity significantly reduces when a high current is passed. On the other hand, the demand for a further improvement of the performance of a lithium secondary battery mentioned above, i.e., the demand for early provision of a lithium secondary battery having further improved performance of the positive electrode and charging/discharging characteristic has been becoming strong. Under these circumstances, in particular in relating to the lithium metal oxide used as a positive electrode active material of a lithium secondary battery, early provision of a lithium metal oxide that has more excellent performance including the shape of the particle is desired strongly.